Monday Pharmaceutical Mystery: June 10


Why is a patient being prescribed an unusually high dose of loratadine?

KJ is a patient, age 45 years, at your pharmacy. She brings in a prescription for ranitidine 150 mg PO BID and asks you to help her find the largest size box of OTC loratadine. She says that her allergist told her to take loratadine 10 mg, 3 times daily. You tell KJ that it must be a mistake, because loratadine 10 mg is dosed once daily.

She pulls out a note from her allergist, and sure enough it reads, “loratadine 10 mg: take one tablet 3 times daily.”

Mystery: Why is KJ taking an unusually high dose of loratadine?

Solution: Upon further conversation, KJ tells you that she has mast cell activation syndrome (MCAS). Mast cells, part of the immune system, react to foreign bodies/injury by releasing chemical mediators, such as histamine, when activated. In a healthy person, these chemicals protect and heal the body, but in a patient with MCAS, these chemicals are inappropriately triggered and released. Patients may be triggered by foods, exercise, chemicals, fragrances, stress, or other triggers, and often experience new triggers over time.

MCAS may affect many body systems and cause symptoms such as skin rashes (often severe), pain, gastrointestinal problems, mental health issues, and anaphylaxis, among others. MCAS can present along with Ehlers Danlos Syndrome (EDS), and/or Postural Orthostatic Tachycardia Syndrome (POTS).

Treatment is very individualized to the patient’s symptoms. Treatment often includes trigger avoidance, mast cell stabilizers, and/or high doses of antihistamines/H2 antagonists to block histamine from H1 and H2 receptors. Many patients take loratadine (or another H1 antagonist) along with ranitidine (or another H2 antagonist) to help combat mast cell activation.


Mast Cell Action. About MCAs. Mast Cell Action website. Accessed June 9, 2019

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